The present invention relates to a portable 4-cycle engine used for driving a portable machine such as a back pack blower or a bush cutter, and to the portable machine equipped with the 4-cycle engine.
The present application claims priority from Japanese Patent Application Nos. 2006-012876 and 2006-012877, the disclosures of which are incorporated herein by reference.
Combustion chamber shapes for ideal 4-cycle engine performance have been studied in order to improve the output of the engine, or to reduce harmful exhaust-gas emissions. For example, the wedge-shaped combustion chamber and the bathtub combustion chamber are known (see Japanese Examined Patent Publication No. Sho 60-5774). Some large 4-cycle engines for use in vehicles or the like have a separate cylinder and cylinder head to achieve a more appropriate combustion chamber shape.
On the other hand, portable 4-cycle engines for use in back pack blowers, bush cutters and the like conventionally have a cylinder head integral with the cylinder because their top-priority requirements are small size and light weight (see, for example, Japanese Patent No. 3159296).
One problem with the engine having a separate cylinder and cylinder head, however, is that the cylinder head must be firmly and air tightly fastened to the cylinder, because otherwise the combustion gas will leak through the interface, resulting in a power loss. Sometimes combustion gas may leak through the interface resulting from thermal expansion of the cylinder head due to a sharp temperature gradient which occurs in the cylinder head where the combustion chamber is formed. Such gas leak causes an engine power loss, and contamination or damage of the engine. Accordingly, improvements in the cooling performance and rigidity of the cylinder head are the crucial issues in designing engines with a separate cylinder and cylinder head.
One of the portable machines, for example, back pack blowers may be operated at length at a high speed of 8,000 rpm, and also bush cutters may be operated in an rpm range of 7,000 to 10,000 rpm. The cylinder and the cylinder head are subjected to high-frequency impact during the operation, and also the cylinder head is subjected to direct impact from the valve train components in the valve train area inside the cylinder head. Therefore, the cylinder head in particular needs to have high rigidity in order to prevent deformation or damage of the cylinder head and to maintain the seal between the cylinder and the cylinder head. Increasing the thickness of the cylinder head or the height of the bosses provided for bolt-tightening with the cylinder leads to an increase in the size and weight of the engine, which is incompatible with the top-priority requirements that the portable machine should be small and light.
For these reasons, portable machines are usually not equipped with 4-cycle engines with a separate cylinder and cylinder head. On the other hand, conventional 4-cycle engines with a one-piece cylinder and cylinder head assembly have reached their limitations in enhancing the engine power output, and portable machines equipped with such engines have a problem of poor operability due to the low engine power output.
The present invention was devised in view of the above problems in the conventional techniques and its object is to provide a portable 4-cycle engine with a small, lightweight, high-power, and high-durability cylinder head for use in portable machines such as back pack blowers, bush cutters and the like which are required to be small, light, and powerful.
Another object of the invention is to provide a portable machine equipped with a 4-cycle engine with improved engine cooling performance, operation efficiency, and maintenance accessibility.